1. Field
One or more aspects of one or more embodiments of the present disclosure relate to an organic light-emitting device emitting delayed fluorescence.
2. Description of the Related Art
In an organic light-emitting device (OLED), holes provided from an anode and electrons provided from a cathode are combined in an organic emission layer formed between the anode and the cathode to generate light. Such OLED devices typically have good color reproduction properties and color purity, short response times, self-emitting characteristics, small thickness, light-weight properties, high contrast ratios, side viewing angles, low driving voltages, and/or low power consumptions. Due to these properties, OLEDs are widely used for television sets, computer monitors, mobile communication terminals, MP3 players, automobile navigation devices, and a variety of other devices.
An organic light-emitting device generally includes a substrate, an anode, a hole transport layer, an emission layer, an electron transport layer and a cathode that are sequentially stacked in the stated order. When a voltage is applied between the anode and the cathode, holes provided from the anode are injected into the emission layer through the hole transport layer, and electrons provided from the cathode are injected into the emission layer through the electron transport layer. In the emission layer, the holes and the electrons are recombined to produce excitons, which then radiatively decay to emit light having a wavelength corresponding to a band gap of the material that constitutes the emission layer.
Materials for forming the emission layer may be classified into a fluorescent material using a singlet state S1 emission mechanism and a phosphorescent material using a triplet state T1 emission mechanism. These luminescent materials may be used alone or may be doped on a host, and a generation ratio of a singlet exciton to a triplet exciton in the emission layer is statistically 1:3.
Recently, organic light-emitting devices using delayed fluorescence have been actively developed, in addition to organic light-emitting devices using fluorescence emitted from an excited singlet state, or using phosphorescence emitted from an excited triplet state. Delayed fluorescence refers to a fluorescent emission that activates an energy up-conversion from a lower energy excited triplet state to a higher energy excited singlet state. Since delayed fluorescence is emitted from the singlet state via the triplet state, delayed fluorescence, in general, has a long lifespan.
For the ease of energy up-conversion from the triplet state to the singlet state, it is advantageous for the luminescent material to have a relatively small energy difference between the triplet state and the singlet state. In addition, when a luminescent material acts as a dopant, its ability to maximize the conversion of the triplet excited state into the singlet excited state may be influenced by the triplet energy level of a host. For example, when the host has a high triplet energy level, charges may not be effectively injected from adjacent layers due to the host's large band gap energy, and because of the host's short conjugation length, charge transport characteristics may decrease.